Eventually this will be a MELPA package, but for now, just clone the repository somewhere on your machine and do M-x load-file <path to ox-pmwiki>.el
ox-pmwiki plugs in to the normal org export framework. Assuming you’ve loaded it, it should show up in the org-export dispatcher (accessed via C-c C-e) with the access key w.
The major limitation thus far is a lack of table support. Everything else, such as text formatting, bulleted and numbered lists, headings, etc, mostly works.